JPS62253217A - Isolation circuit with no adjustment - Google Patents

Abstract

PURPOSE:To keep the circuit gain constant with no adjustment, and to send an input signal to the output side with high accuracy by providing a current feedback bias setting current circuit in parallel with a signal output terminal to a photocoupler output terminal. CONSTITUTION:When the DC transfer ratio of a photocoupler 9 is increased, the emitter potential of a photo transistor (TR) 18 rises, the base potential of a TR 26 is increased by resistors 24, 25 and its collector current is increased. Then the current flowing to the emitter resistor 23 is increased, the collector- emitter voltage of a TR 26 is decreased by the current negative feedback actioin by the resistor 23 and an output signal voltage applied to an output terminal 15 is lowered. When the transfer ratio is small conversely and the emitter potential of the TR 18 is lowered, the base potential of the TR 26 is lowered, the collector current is decreased, the collector-emitter voltage of the TR 26 rises by the resistor 23 and the voltage to a terminal 15 rises. Since the imput impedance of the TR 26 is larger than the collector-emitter resistor of the TR 18, the circuit gain is constant independently of the DC transfer ratio of the photocoupler.

Description

[Detailed Description of the Invention] [Industrial Field of Application] This invention relates to an isolation circuit for electrically insulating a human input signal and an output signal using a photocoupler, and in particular, the invention relates to an isolation circuit for electrically insulating a human input signal and an output signal using a photocoupler. The present invention relates to an unadjusted isolation circuit that can always provide a constant circuit gain even when the circuit is in use.

[Prior Art] In recent years, isolation circuits using photocouplers have been used in televisions and the like to electrically isolate audio signals and remove noise contained in the signals.

FIG. 2 is a circuit diagram showing the configuration of an insulation circuit using a conventional photocoupler. In FIG. 2, an isolation circuit using a conventional photocoupler (hereinafter simply referred to as a photocoupler isolation circuit) includes a photocoupler 9 that receives an input signal.
It is divided into a secondary side and a secondary side that outputs an output signal. Photocoupler 9 includes a light emitting diode 17 that converts a given electrical signal into an optical signal, and a phototransistor 18 that converts the optical signal from light emitting diode 17 into an electrical signal.

The primary side includes a pnp transistor 8 that amplifies the input signal voltage applied via the input terminal 2, converts it into a current signal, and applies it to the light emitting diode 17.

Between the input terminal 2 and the base of the bipolar transistor 8 is a coupling capacitor 3 that removes the direct current component of the human input signal applied via the input terminal 2 and transmits it, and an electric signal applied via the coupling capacitor 3. A base current supply resistor 4 is provided to receive the current and transmit it to the base of the transistor 8. The transistor 8 is
Its collector is connected to the cathode of the light emitting diode 17, its emitter is grounded via an emitter resistor 7 for current feedback, and its base receives an input signal via a resistor 4. Furthermore, resistors 5 and 6 are connected to the base of the transistor 8 for dividing the voltage from the primary power supply l and applying a bias potential to the base of the transistor 8. The collector current flowing through transistor 8 is determined by the values of resistors 5 and 6.7. A light emitting diode 17 that generates an optical signal in response to the collector current from the transistor 8 has its anode connected to the positive electrode of the primary power supply 1 and its cathode connected to the collector of the transistor 8 .

The secondary side includes a phototransistor 18 that converts the optical signal from the light emitting diode 17 into an electrical signal and outputs it from its emitter, and an npn transistor 14 that supplies an output signal voltage in response to the emitter output of the phototransistor 18.
including. The phototransistor 18 has its emitter connected to the base of the transistor 14 via the resistor 11, is grounded via the resistor 10 and the variable resistor 13, and has its collector connected to the positive electrode of the secondary power supply 16. . The transistor 14 has its collector connected to the positive electrode of the secondary power supply 16, its emitter grounded via the emitter resistor 12, and receives the output of the phototransistor 18 via the resistor 11 at its base. The emitter output of the transistor 14 becomes the output signal voltage applied via the output terminal 15. Resistor 10.13 is transistor 1
This is a resistor for bypassing the base current applied from the phototransistor 18 to the base of variable resistor 1.
By adjusting the value of 3, the base current applied to the base of transistor 14 is adjusted.

Next, the operation will be explained. The input signal applied via the input terminal 2 has its DC component removed by the coupling capacitor 3, and then is applied to the base of the transistor 8 via the resistor 4. The input signal applied via the resistor 4 is amplified and converted into a current signal, and the current signal is applied to the light emitting diode 17. The collector current flowing through transistor 8 is determined by the value of resistor 5.6.7. The input signal current amplified by transistor 8 flows through light emitting diode 17 . The light emitting diode 17 generates an optical signal in response to the collector current of the transistor 8 and applies it to the base of the phototransistor 18 . Phototransistor 18 supplies an emitter current to resistor 11 and resistor 10.13 in response to an optical signal from light emitting diode 17 applied to its base. The current output from the emitter of the phototransistor 18 is connected to the resistor 11.
is applied to the base of transistor 14 through resistor 10.13, and is bypassed through resistor 10.13. Transistor 14 causes an emitter current to flow in response to a base current applied through resistor 11 and applies it to resistor 12 . The emitter potential of the transistor 14 is applied via the output terminal 15 to output an output signal.The emitter potential of the phototransistor 18 changes depending on the value of the variable resistor 13, and the emitter current from the transistor 14 changes accordingly. By adjusting the resistance value of this variable resistor 13, the circuit gain (the input signal voltage given through the input terminal 2 and the output terminal 15
The ratio between the output signal voltage and the output signal voltage provided via the output signal voltage is adjusted.

[Problems to be Solved by the Invention] The conventional photocoupler isolation circuit is configured as described above, and the DC current transfer ratio of the photocoupler 9 (the current flowing through the light emitting diode 17 and the current flowing through the phototransistor 18) is ratio) differs depending on the product, for example 0.5
.about.6, the drawback is that the circuit gain must be adjusted by adjusting the resistance value of the variable resistor 13 for each photocoupler used.

Therefore, an object of the present invention is to provide a non-adjustable isolation circuit that can eliminate the above-mentioned drawbacks and always realize a constant circuit gain regardless of the DC current transfer ratio of the photocoupler.

[Means for Solving the Problems] The non-adjustable insulation circuit according to the present invention has a current feedback bias type constant current circuit connected to the emitter terminal of a phototransistor in parallel with a signal output terminal.

[Function] In the constant current circuit according to the present invention, when the output potential of the phototransistor increases, the value of the current flowing therein increases,
The operation of the current negative feedback bias lowers its output voltage, and on the other hand, when the output potential of the food transistor decreases, its current value decreases, and the current negative feedback bias effect increases its output potential, keeping the output signal voltage constant. Make it.

[Embodiment of the Invention] FIG. 1 is a circuit diagram showing the configuration of an unadjusted insulation circuit according to an embodiment of the invention. In FIG. 1, a photocoupler 9 divides the device into a primary side that receives an input signal and a secondary side that outputs an output signal. The photocoupler 9 includes a light emitting diode 1 that generates an optical signal in response to a given signal current.
7, and a phototransistor 18 that generates an electrical signal in response to the optical signal from the light emitting diode 17.

The primary side is provided with a light emitting diode 17 connected in the forward direction to the path of the primary side power supply 1, and a resistor 22 connected in series with the light emitting diode 17 for applying a bias current to the light emitting diode 17. . A human input signal, for example an audio signal, applied via the input terminal 2 is connected to a resistor 4, a capacitor 21 connected in parallel with the resistor 4, and a capacitor 21 for improving high frequency response characteristics to the input signal.
It is connected in series with the parallel body of the capacitor 21, and is supplied to the connection point between the cathode of the light emitting diode 17 and the resistor 22 via the coupling capacitor 3 for removing DC components.

On the secondary side, a current feedback bias type constant current circuit is connected in parallel between the emitter terminal of the phototransistor 18 and the output terminal 15. The constant current circuit is an npn that serves as a constant current source.
includes a transistor 26;

The collector of the transistor 26 and the emitter of the phototransistor are grounded via an emitter resistor 23 for negative current feedback, and the base thereof is a resistor 25 for applying a bias potential.
Connected to 24 connection points.

One terminal of the resistor 25 is connected to the emitter of the phototransistor 18 and the collector of the transistor 26 and the output terminal 15, and the other terminal is connected to the base of the transistor 26. One terminal of the resistor 24 is connected to the base of the transistor 26 and the other terminal of the resistor 25, and the other terminal is grounded. The current flowing through transistor 26, that is, the collector current of transistor 26, is determined by the resistance values of resistors 23, 24, and 25.

Here, the output signal voltage output to the output terminal 15 is determined by the collector-emitter voltage of the transistor 26, and is set to be approximately 1/2 of the voltage of the secondary power supply 15. Next, the operation will be explained.

A human power signal is applied via the input terminal 2, and is applied to the cathode via the resistor 4, capacitor 21, and capacitor 3. Since a DC bias current is applied to the light emitting diode 17 by the resistor 22, this bias current is amplitude-modulated by the input signal and is applied to the light emitting diode 17. The light emitting diode 17 generates an optical signal in response to this input signal and applies it to the base of the phototransistor. The phototransistor 18 becomes active in response to an optical signal from the light emitting diode 17, and outputs a signal from its emitter. An electric signal appearing at the emitter of the phototransistor 18 supplies current to the base of the transistor 26 through the resistor 25, and the transistor 26
A collector current flows through the transistor 26, and a voltage between the collector current and the collector current is generated in the transistor 26, and a direct current that becomes approximately 1/2 of the voltage of the power supply 16 (actually, the voltage drop at the emitter resistor 23 is also added) is generated at the output terminal 15. An output signal having a voltage is output.

Now, when the DC current transfer ratio of photocoupler 9 increases,
The emitter potential of the phototransistor 18 increases,
Accordingly, the base potential of the transistor 26 increases due to the resistors 25 and 24, and the collector current flowing through the transistor 26 increases. When this collector current increases, the current flowing through the emitter resistor 23 also increases, and due to the current negative feedback effect of this resistor 23, the collector-emitter voltage of the transistor 26 decreases, and the output signal voltage (DC voltage) applied to the output terminal 15 decreases. ) decreases.

Conversely, when the DC current transmission ratio of the photocoupler 9 is small and the output potential appearing to the emitter of the phototransistor 18 decreases, the base potential of the transistor 26 decreases accordingly, and the collector current flowing through the transistor 26 decreases.
Due to the current negative feedback effect of the resistor 23, the collector-emitter voltage of the transistor 26 increases, and the output signal voltage applied to the output terminal 15 increases. As a result, regardless of the DC current transfer ratio of the photocoupler 18, the output terminal 15
The DC voltage level applied to is almost constant.

Furthermore, since the input impedance of the constant current circuit made up of the transistor 26 is sufficiently large compared to the collector-emitter resistance of the phototransistor 18, the circuit gain is constant regardless of the DC current transfer ratio of the photocoupler 9.

Now, as an example, in the circuit configuration shown in FIG.
The voltage of power supplies 1 and 16 is 12V, and the resistance value of resistor 4 is 3.9.
Ω, the capacitance value of capacitor 21 is 5600 pF, the capacitance of capacitor 3 is 10 μF, the resistance value of resistor 22 is 5.6 Ω, the resistance value of resistor 23 is 1.8 Ω, the resistance value of resistor 24.25 is both The circuit characteristics when 39Ω is set to Ω and a small signal amplification transistor is used as transistor 26 are experimentally determined as follows: DC potential of output terminal 15 28.2±1v Circuit gain:
It becomes 1.0±1%.

The larger the amplification factor of the transistor 26, the greater the negative feedback effect by the resistor 23, and the variation in the DC voltage at the output terminal 15 caused by the variation in the DC current transfer ratio of the photocoupler 9 is eliminated. The dynamic range is 8Vp.
-p and is usually used at 1.4Vp-p or less, providing sufficient characteristics.

Regarding the circuit gain, the collector-emitter resistance of the phototransistor 18 of the photocoupler 9 is usually about 5
Ω, the input impedance of the constant current circuit made up of the transistor 26 is 50Ω or more. It is hardly affected.

Note that in the above embodiments, a constant current circuit consisting of a transistor, a resistor for negative current feedback connected to the emitter of the transistor, and a resistor for applying a bias potential to the base of the transistor is shown. Even with other circuit configurations, for example, a configuration using an OP amplifier, the same effects as in the above embodiment can be obtained as long as the circuit is a constant current circuit in which negative feedback is provided by current.

Although the case of using an audio signal as an input signal has been described, the same effects as in the above embodiment can be obtained also when a video signal is used as an input signal.

[Effects of the Invention] According to the present invention, the photocoupler insulating circuit is configured to receive the photocoupler output potential using a current feedback bias type constant current circuit.
There is no need to use a variable resistor for circuit gain adjustment, the circuit gain can be easily maintained constant without adjustment with a simple circuit configuration, and an isolated circuit that can accurately transmit input signals to the output side is obtained. It will be done.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing the configuration of an unadjusted insulation circuit according to an embodiment of the present invention. FIG. 2 is a circuit diagram showing the configuration of an insulation circuit using a conventional photocoupler. In the figure, 9 is a photocoupler, 17 is a transistor serving as a light emitting dummy current source, 23 is a current feedback circuit, a mitter resistor, and 24.25 is a bias resistor. In addition, in the figures, the same reference numerals indicate the same or corresponding parts.

Claims (4)

[Claims] (1) In an unadjusted isolation circuit that uses a photocoupler to electrically isolate a signal applied through an input terminal and outputs it from a signal output terminal, a current is applied to the photocoupler output terminal in parallel with the signal output terminal. A non-adjustable isolation circuit characterized by a feedback bias type constant current circuit. (2) The non-adjustable insulation circuit according to claim 1, wherein the constant current circuit receives the photocoupler output with a high resistance. (3) The constant current circuit includes: a bipolar transistor whose one conducting terminal is connected to the photocoupler output terminal and the signal output terminal, and whose other conducting terminal is grounded via a first resistance element; a second resistance element connected between the one conduction terminal and the control electrode of the bipolar transistor; and a second resistance element connected in series with the second resistance element between the control electrode of the bipolar transistor and a ground potential. The non-adjustable insulation circuit according to claim 1 or 2, comprising a third resistance element. (4) The non-adjustable insulation circuit according to claim 3, wherein the resistance value of the first resistor is set to be smaller than the resistance values of the second and third resistors.